The present invention relates to video encoding and decoding systems and particularly concerns a video compression system employing adaptive sub-band coding.
Adaptive sub-band coding is a process which has been frequently proposed for compressing a wide band video signal to enable its transmission through a channel of limited bandwidth, such as a standard 6 MHz television channel. See, for example, the article by Paul A. Wintz entitled "Transform Picture Coding" in the Proceedings of the IEEE, Vol. 60, No. 7, pages 809-820, July, 1972. Sub-band coding as the term is used herein, refers generally to a process wherein a video image signal is converted into a plurality of spectral coefficients representative thereof and may be effected either by spatially filtering the video signal or by subjecting it to a suitable block transform, such as the discrete cosine transform (DCT). In either case, the video signal is separated into a plurality of sub-bands each comprising a series of coefficients, with the coefficients derived for each sub-band representing a different spectral component of a respective portion of the image corresponding to the video signal.
The sub-band coefficients are subsequently processed in accordance with a data compression algorithm whereby the coefficients are normally quantized or truncated based on the source statistics of typical video image signals. Use of these types of algorithms is typically referred to as "source coding". In particular, since it has been found that most of the video information is contained in the lower order sequency domain coefficients, these coefficients are normally quantized to provide relatively high resolution, while the higher order sequency domain coefficients are quantized to provide relatively low resolution. In the cases where coefficient truncation is effected, the lower order coefficients are truncated less heavily than the higher order coefficients.
Thus, for example, the lowest order coefficients may be quantized with eight-bit resolution, the next highest order coefficients with six-bit resolution, and so on. While this technique achieves a certain degree of compression, it is often necessary to further compress the coefficients before transmission. Further compression may be achieved by deriving a so-called "activity function" which provides a measure of the AC energy in respective portions of the video image. Since larger errors can be tolerated in portions of the video image producing a large activity function, the degree of quantization in these areas can be reduced, thereby further reducing the amount of data required for transmission. For example, if a particular portion of the video image is characterized by a relatively large activity function, the quantization of the lowest order coefficients derived therefor may be reduced from eight bits to six bits, the next highest order coefficients from six bits to four bits, and so on.
Adaptive sub-band coding systems of the foregoing type can achieve substantial levels of data compression, but often at the expense of producing undesired artifacts in the reproduced video image. These artifacts are largely the result of the arbitrary nature in which the data compression algorithms effect quantization of the sub-band coefficients. In particular, the use of prior art source coding quantization algorithms results in the over and under quantization of the various coefficients to such an extent that the information represented by the transmitted coefficients is inadequate to enable reproduction of the video image without significant visible artifacts.
The present invention provides an improved system for variably quantizing or truncating the sub-band coefficients in a manner such that visible artifacts in the reproduced video image are greatly minimized. Tee system is based on perceptual coding criteria which may be used to quantize or truncate the sub-band coefficients in conformance with the response of the human eye to different classifications of video images.